Void replenishment: how voids accrete matter over cosmic history

Cosmic voids are underdense regions filling up most of the volume in the Universe. They are expected to emerge in regions comprising negative initial density fluctuations, and subsequently expand as the matter around them collapses and forms walls, filaments and clusters. We report results from the analysis of a cosmological simulation specially designed to accurately describe low-density regions, such as cosmic voids. Contrary to the common expectation, we find that voids also experience significant mass inflows over cosmic history. On average, $10\%$ of the mass of voids in the sample at $z \sim 0$ is accreted from overdense regions, reaching values beyond $35\%$ for a significant fraction of voids. More than half of the mass entering the voids lingers on periods of time $\sim 10 \, \mathrm{Gyr}$ well inside them, reaching inner radii. This would imply that part of the gas lying inside voids at a given time proceeds from overdense regions (e.g., clusters or filaments), where it could have been pre-processed, thus challenging the scenario of galaxy formation in voids, and dissenting from the idea of being pristine environments.